A medical intervention, for example the ablation of diseased cardiac tissue to treat cardiac arrhythmia is generally carried out by means of a medical instrument—for example a catheter—inserted into the patient's body. During the treatment the treating physician then generally does not have direct sight of the instrument and the body region to be treated. Instead, in order to be able to carry out the treatment precisely the instrument is normally made visible within the patient's body by means of an imaging method. The imaging methods used for this purpose are in particular x-ray imaging methods. The instrument can then be tracked for example in an x-ray image of the patient.
Additionally or alternatively an instrument location facility (also referred to as a tracking system) is frequently deployed to determine instrument position. Such a tracking system captures the position of the instrument in space by means of an optical, acoustic, electromagnetic or impedance-based method. To this end detection points defined on the instrument are frequently provided, which can be detected by the tracking system.
Imaging apparatuses with real-time resolution, e.g. x-ray fluoroscopy devices, generally only provide a two-dimensional but therefore relatively precise image of the instrument. With a tracking system however the spatial position of the instrument can be determined continuously in real time, although generally only locally; there is no overview of the entire object or environment. Also the position determination of the tracking system is frequently inaccurate. In particular a tracking system often shows the instrument position in a distorted manner.
In order to be able to make better use of the respective advantages of imaging and instrument tracking, the coordinate systems of the imaging apparatus and the tracking system are superimposed, so that a corresponding point in the image coordinate system can be determined for each point in the tracking coordinate system. For example the position information from the tracking system can then be used to display the medical instrument in a three-dimensional x-ray image produced beforehand by imaging, in particular a computed tomogram or angiogram, and to navigate it in this virtual 3D image of the patient.
The superimposition or mapping of the tracking coordinate system with the image coordinate system is also referred to as registration. To carry out such registration a so-called calibration phantom is generally used, which contains a large number of detectable marking points, whose relative positions in respect of one another are known. The calibration phantom is in a fixed position in space, for example under a patient table, and it determines the location of the marking points thus established both by means of imaging and also by means of the tracking system. The captured coordinates of the marking points in the tracking coordinate system and in the image coordinate system are then used to determine a transformation rule, which maps the tracking coordinate system onto the image coordinate system.
However such a registration is relatively complex to carry out and sometimes disrupts the ongoing operation of the associated medical facility.